In that case it would copy any text string, strings which are not urls too. It defeats the purpose of using mime data in that case.
I actually wish to monitor http/ftp download urls copied to clip board, and later on filter out urls of webpages

The problem here is that it depends on the application thatcopies the text ton the clipborad which type the clipboard data has. If it copies it as text, you can only read it as text. But you could use QUrl to verify, whether it is a url.